The food processing industry has a need for dewatering food product—for removing food product from liquid or water. For example, processing food such as poultry requires a large amount of water. Water, after being used to process poultry, often includes small pieces of food product. It is desirable to separate as much of the food product from the water as possible. Recovered solids can be used or sold for rendering. Recovered water can be reused upstream. However, recovered water can plug nozzles when reused. To avoid plugging nozzles it is desirable to remove particles in the water. Thus, there is a need to dewater food product. Processing food, as used herein, includes cooking, blanching and cooling food product, and cleaning, treating with liquid or water, and reclaiming liquid or water to clean, cook, blanch or cool food product. Food product, as used herein, includes food, raw materials processed into food, and waste generated while processing food.
One prior art technique for removing solids from liquid or water is a dissolved air flotation (DAF) system. DAF systems can be expensive, and can use chemicals that result in recovered solids that cannot be used for rendering.
Rotary drum screens for dewatering food product are known in the prior art. Generally, food and water, or a food slurry, enters a first end of a rotary drum wrapped circumferentially with a screen. The drum is mounted in a frame and rotated. As the drum is rotated, water passes through the screen, and the food product is dewatered. The dewatered product exits from the second end of the drum.
The construction of the screen for a rotary drum is such that the screen is rigid and self supporting. For example, prior art rotary drum screens are typically perforated metal plates or comprised of wedge wire. They can screen particles as small as 0.020 inches (about 500 microns). Some screens also have slot-like openings through which narrow things such as hair can pass. These screens cannot be made to screen smaller particles because their self-supporting construction does not allow for smaller openings. Rigid screens rotate with drum because the screen is part of the support structure.
Mesh screens can strain very small particles 0.005 inches (125 microns) or 0.008 inches (200 microns). Screens of this opening size use very small wires in a mesh weave, and are very flexible and somewhat fragile. Applications requiring fine screening have typically been done with stretched flat screen. The screen is vibrated in a circular fashion, or the screen is on an incline to allow the liquid and solids to flow down a slope. Retention time and effectiveness is controlled by length of the screen and the angle of inclination.
Mesh screens have not been well-suited for rotary drums because they do not have sufficient strength to be self-supporting and must be supported by some external means. Different methods have been used in the prior art to provide the needed support, including wrapping the wire mesh around a heavier structural perforated plate or other supporting structure, and welding or clamping it into place. The perforated plate provides the structure, and the fine mesh provides the necessary resolution to screen small particles.
Such prior art rotary mesh screens suffer from several deficiencies. First, the perforated plate or other supporting structure makes the screen difficult to clean. Also, the percent open area is greatly reduced by the supporting structure, thus reducing the amount of water that can be removed—current waste water screening with a fine screen is limited to several hundred gallons per minute. Third, because they are mounted to a perforated plate, it is difficult to change screens to accommodate a different size opening. These drawbacks have limited the use of fine wire screens.
Cleaning fine mesh screens has been difficult, and spray nozzles have been used to back flush the screens to make them function for a reasonable period of time. Flat stretched screens are cleaned using a variety of methods, including using an array of nozzles under the entire screening surface. This requires extensive use of cleaning water, and detracts from the capacity of the screen. Some inclined flat screens use moving sprays above, and/or below the screen to shear off the material. This increases mechanical complexity and service and maintenance costs. Others use a moving belt which is difficult to clean.
Recovery of waste water in a food plant is becoming an economic necessity in many food industries. Thus, a design is needed that is of a more sanitary design, will be able to rapidly change screen sizes for different screening applications and have a more effective open area, all without damaging the fragile screen material while providing support for the mesh material. Accordingly, it is desirable to have a rotating drum screen for use in food processing that is comprised of a small opening, such as a fine wire mesh, yet able to be supported without adversely limiting the amount of dewatering able to be accomplished. Preferably, the screen is relatively easy to clean, and allows for rapidly change screen sizes for different screening applications and have a more effective open area, all without damaging the fragile screen material while providing support for the mesh screen.